1. Field of the Art
The present invention relates to the treatment of a substrate so that it is suitably coated with a conductive material, by chemical or electroless plating technique, to produce a printed-wiring board having an improved wiring quality.
2. Description of the Prior Art
In the art of forming a printed wiring pattern on a printed-wiring board by applying a chemical or electroless plating layer of metal to a starting material, i.e., to an electrically insulating substrate or base, it is generally practiced to apply a catalyst to the insulating base so as to start an electroless plating reaction on the surface of the insulating base. To this end, palladium or other noble metals are added as such a catalyst to the insulating material, prior to the electroless plating of the insulating base. However, the mere addition of the catalyst does not permit an electroless plating layer to be formed with excellent heat resistance, and high adhesion to the insulating base. These are physical properties required of a printed-wiring board.
In view of the above requirements, a catalyst for electroless plating is traditionally applied to an insulating base for a printed-wiring board, by employing one of methods shown in FIGS. 1-4. The first method is illustrated in FIGS. 1 and 2, wherein a prepreg is prepared by pre-impregnating cellulose or craft paper with a varnish admixed with a complex compound containing palladium as a catalyst 2. Plural prepregs prepared in this manner are laminated into an electrically insulating base 3. The thus obtained insulating base 3 is coated with layers 4 of an adhesive consisting essentially of a mixture of denaturated phenolic resin and nitrile rubber which contains catalytic cores 5 of a metal oxide. Prior to applying this adhesive layer 4 to the insulating base 3, the adhesive is mixed with catalytic cores 5 of a metal oxide which are soaked with an activating solution of tin chloride (stannous chloride), palladium chloride, hydrochloric acid and other activating agent. The insulating base 3 and the adhesive layers 4 containing the catalytic cores 5 constitute a processed substrate 1 which is subsequently subjected to electroless plating. Thus, both of the insulating base 3 and the adhesive layers of 4 of the processed substrate 1 contain the catalyst. However, the concentration of the catalyst adjacent to the surface of the substrate 1 is not sufficiently high, i.e., low particularly at the inner surface defining a through-hole formed in the substrate 1, and therefore the substrate tends to suffer the generation of blow holes. To avoid this drawback, it is necessary to additionally apply the catalyst 2 to the inner surface of a through-hole on the substrate by using a solution of tin (stanneous) chloride, palladium chloride and hydrochloric acid, before the surfaces of the adhesive layers 4 are roughed for exposing the catalytic cores 5.
An alternative known method is shown in FIG. 3, wherein substantially the same steps as used in the method of FIG. 1 are taken to produce a processed substrate 1 illustrated in FIG. 4, which is different from the substrate of FIG. 2 only in that the substrate prepared with the alternative method does not contain the catalyst 2 within the insulating base 3. Like the preceding method, however, this alternative method also includes the step of applying the catalyst 2 to the surfaces of the adhesive layers 4 by using an activating solution of tin (stanneous) chloride, palladium chloride and hydrochloric acid.
The above-discussed known methods and the processed substrates prepared with these methods, which are used to produce metal-plated boards for printed-wiring boards, have the following inconveniences:
(1) Since the crystallization of a metal to be applied to the substrate 1 by electroless plating requires the presence of the catalyst 2 and the catalytic cores 5 only at the skin of the adhesive layers 4 applied to the insulating base 3, the catalyst 2 and catalytic cores 5 which are present in the insulating base 3 and in the interior of the adhesive layers 4 according to the known methods are not necessary, and cause an increase in cost of manufacture of the end product, i.e., a printed-wiring board produced from the substrate 1 plated with a conductive material. Further, the catalyst and catalytic cores within the substrate 1 will lead to deterioration of the properties of the printed-wiring board.
(2) The known methods use as the catalytic cores 5 solid solutions of oxides, for example, ground powder of ZrSiO.sub.4 (zircon), SiO.sub.2 (silica), Al.sub.2 O.sub.3 (alumina), TiO.sub.2 (titania) and Al.sub.2 Si.sub.2 O.sub.5 (OH).sub.4 (kaolinite). The size of such powder is as large as approx. 2-10.mu.. In fact, it is preferred that the catalytic cores 5 be fine particles, for example, less than 1.mu.. In other words, the known processed substrate 1 containing the catalytic cores 5 of relatively large size is more likely to have a rough surface, which degrades the resolution of a wiring pattern formed by printing or dry film technique, and reduces crystallinity of the electroless plating layer which forms, or a portion of which is left to form a wiring pattern.
(3) Since the catalytic cores 5 are admixed with the adhesive 4, and applied to the insulating base 3 in the form of the adhesive layers 4, the distribution of the cores 5 over the surfaces of the adhesive layers 4 is not satisfactory, and it is very difficult to attain a sufficient concentration of the catalytic cores 5 on the surfaces of the adhesive layers 4. On the contrary, increasing the concentration of the catalytic cores 5 within the adhesive layers 4 will result in degrading the properties of the adhesive. Thus, the known methods have such a dilemma. In the case where an electroless plating is applied to the known substrate 1 with the catalytic cores 5 contained in the manner described above, the plating layer generally has a low peeling strength (flake-off resistance), i.e., 1.2-2.0 Kg/cm at 25.degree. C. The peeling strength at elevated temperatures is abruptly lowered to 0.3-0.7 Kg/cm. In other words, a printed-wiring board prepared from the known processed substrate 1 has a low soldering heat resistance, and consequently may suffer a trouble that the wiring pattern is easily separated or peeled off from the substrate when the components on the board are replaced.
(4) The previously described additional application of the catalyst 2 in an activating solution causes the reduction in insulation resistance of the substrate 1. Further, if there exists an excess of the catalyst 2 which is not firmly bonded to the catalytic cores 5, the adhesion of a subsequently applied electroless plating layer to the insulating base 3 will be reduced.
(5) The adhesiveness of the electroless plating layer relative to the insulating base 3 depends largely on the result of surface roughing operation which is effected for the purpose of exposing the catalyst 2 and catalytic cores 5 existing adjacent the surfaces of the adhesive layers 4 on the insulating base 3. Accordingly, it is difficult to suitably control the surface roughing operation for obtaining an electroless plating layer of consistent quality. The roughing operation may induce another problem that drips of a roughing solution cause the extension of a plating layer in an area in which a wiring pattern is not intended to be formed.
(6) The known processed substrate 1 requires the use of a specially designed insulating base and a specific adhesive agent, which complicates the fabrication procedure, increases the variation in quality, and pushes up the cost of manufacture, of the processed substrate.